Lockable overshot

ABSTRACT

Lockable overshot ( 10 ) includes an elongated body ( 12 ), lifting dogs ( 14   a   , 14   b ) and a locking sleeve ( 16 ). Ends ( 18 ) of the lifting dogs ( 14 ) are configured for latching a conventional spearhead point therebetween. The locking sleeve ( 16 ) is retained on the body ( 12 ) and is provided at one axial end with a circumferential wall ( 66 ) and two diametrically opposed gaps ( 68 ). The locking sleeve ( 16 ) is rotatable on the body ( 12 ) between a locked state preventing ends ( 18 ) of the lifting dogs from pivoting away from each other to release a previously latched spearhead point, and an unlocked state where the locking sleeve ( 16 ) allows the lifting dogs ( 14 ) to move so that the ends ( 18 ) can be pivoted away from each other to release a previously latched spearhead point. In this regard, the locking sleeve ( 16 ) is provided at one axial end with a circumferential wall ( 66 ) provided with a pair of diametrically opposed gaps ( 68 ). When the locking sleeve ( 16 ) is in the locked state, the wall ( 66 ) is disposed inside or between ends ( 56 ) of the lifting dogs ( 14 ). This prevents the ends ( 18 ) of the lifting dogs from pivoting away from each other. When in the unlocked state, the gaps ( 68 ) register with the ends ( 56 ) of the lifting dogs ( 14 ) so that the ends ( 56 ) can be pivoted toward each other thereby pivoting the ends ( 18 ) away from each other to release a previously latched spearhead point.

FIELD OF THE INVENTION

The present invention relates to a lockable overshot, typically, thoughnot exclusively, for use in a wireline spearhead system for lowering andretrieving core barrels and other tools and equipment from a downholelocation.

BACKGROUND OF THE INVENTION

A typical overshot comprises a tubular body in which a pair of liftingdogs is pivotally mounted. The lifting dogs pivot about a pin whichattaches the lifting dogs to the body. A first end of the lifting dogsis adapted for catching a spearhead point therebetween. A spring isdisposed between the second opposite ends of the lifting dogs forbiasing the first ends toward each other to a capture position where aspearhead point can be caught. The bias of the spring holds the firstends of the lifting dogs together about a caught spearhead point.

In order to release a captured spearhead point an operator pushes thesecond ends of the lifting dogs together against the bias of the springthereby spreading the first ends of the lifting dogs and releasing thespearhead point. For safety and efficiency reasons it is critical thatthe lifting dogs do not accidentally or prematurely release a caughtspearhead point. For example when retrieving a core barrel from a holeusing an overshot, serious injury or death can occur to an operator ifthe core barrel becomes accidentally released from the overshot whilethe overshot is being suspended near the top of a drill rig. Similarly,very expensive equipment can be lost downhole if it becomes detachedfrom an overshot.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a lockable overshotin which the possibility of the lifting dogs accidentally or prematurelyreleasing a caught spearhead point is reduced.

According the present invention there is provided a lockable overshotincluding at least:

an elongated body;

at least two lifting dogs pivotally mounted to said body, said ling dogshaving respective first ends configured for latching a spearhead pointtherebetween, said first ends pivotally moveable toward each other to alatched position wherein a spearhead point is latched therebetween andpivotally moveable away from each other to a release position where alatched spearhead point is released; and,

a locking sleeve retained on said body and switchable between: a lockedstate where said locking sleeve locks said lifting dogs in said latchedposition and, an unlocked state where said locking sleeve allows saidlifting dogs to move to said release position.

Preferably said lifting dogs have respective second ends opposite saidfirst ends and said lifting dogs are pivotally mounted at a locationintermediate said first and second ends to said elongated body, and atleast a portion of said locking sleeve is disposed inside said secondends when in said locked state.

Preferably said locking sleeve is rotatably retained on said elongatedbody and is switched between said locked state and said unlocked stateby rotating said locking sleeve about a longitudinal axis of saidelongated body.

Preferably said locking sleeve is provided at one axial end with acircumferential wall formed with a gap for each of said lifting dogswherein when in said locked state said wall is located inside saidrespective second ends and when in said unlocked state said second endsare located in said gaps.

Preferably said lockable overshot further includes détente means forreleasably holding said locking sleeve selectively in said locked andunlocked states.

Preferably said détente means includes a biased locking ball extendingfrom an outer surface of said elongated body and first and secondrecesses formed in said locking sleeve for partially receiving saidlocking ball, said first recess disposed to register with said lockingball when said locking sleeve is in said locked state and said secondrecess disposed to register with said locking ball when said lockingsleeve is in said unlocked state.

In one embodiment said circumferencial wall is of constant outerdiameter.

In an alternate embodiment said circumferential wall is formed with areducing outer diameter in a direction toward said lifting dogs.

Preferably said locking sleeve is further provided with a shoulder ofconstant outer diameter adjacent said circumferential wall on a sidedistant said lifting dogs.

Preferably said first and second recesses are in the form of axiallyextending slots.

Preferably said lockable overshot further includes bias means forbiasing said locking sleeve axially in a direction toward said liftingdogs.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described by way ofexample only with reference to the accompanying drawings in which:

FIG. 1 is an exploded view of an embodiment of the lockable overshot;

FIG. 2 is a schematic representation of the lockable overshot when in alocked state;

FIG. 3 is a representation of the lockable overshot when in an unlockedstate; and,

FIG. 4 is a schematic representation of a second embodiment of thelockable overshot.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1–3 of the accompanying drawings, the lockableovershot 10 includes an elongated body 12, lifting dogs 14 a and 14 b(hereinafter referred to in general as “lifting dogs 14”) and a lockingsleeve 16. The lifting dogs 14 are pivotally mounted to the body 12 andare provided with respective first ends 18 configured for latching aspearhead point therebetween. To this end, the lifting dogs 14 are of asubstantially conventional configuration in which each first end 18 isprovided with a hook-like formation which catches beneath a spearheadpoint (not shown). The locking sleeve 16 is retained on the body 12 andis switchable to locked state where the locking sleeve 16 locks thelifting dogs in the latched position preventing them from pivoting awayfrom each other. In this embodiment the sleeve 16 or at least a portionthereof is seated between or inside the lifting dogs 14 to prevent themfrom pivoting to the ends 18 apart sufficiently to allow a latchedspearhead point to be released. In this way, the overshot 10 ispositively locked on to a spearhead point and, except for a materialfailure, can not be released therefrom. This position is depicted inFIG. 2. The locking sleeve 16 can be switched by an operator to anunlocked state where it is disengaged from the lifting dogs 14 allowingthe ends 18 to be pivoted away from each other in a convention manner toa released position (shown in FIG. 3) where a previously caughtspearhead point can be released from between the lifting dogs 14 in aconventional manner.

Looking at the components of the overshot 10 in more detail, it can beseen that the body 12 includes a hollow tubular lower portion 20 towhich the lifting dogs 14 are pivotally mounted, and an integral reduceddiameter upper portion 22. An elongated transversely extending slot 24is formed through the body 12. While a majority of the length of theslot 24 is in the lower portion 20, an upper end 26 of the slot 24 isformed in the upper portion 22. The slot 24 provides room for thelifting dogs 14 to pivot as well as facilitating the assembly of thelifting dogs 14 into the body 12. The outer surface of the lower portion20 is of an octagonal section while the upper portion 22 is formed witha circular section. An upper end of the lower portion 20 tapers to anintermediate portion 28 of the body which is formed of a constantdiameter. The portion 28, is provided with a tapered surface 29 leadingto a stepped face 31 which extends to the upper portion 22.

Extending radially through the lower portion 20 transverse to the slot24 is a hole 30 for receiving a pivot pin 32 which pivotally mounts thelifting dogs 14 to the body 12. In this regard, the pivot pin 32 passesthrough respective eyelets 34 formed intermediate the length of each ofthe lifting dogs 14. A second smaller diameter hole 36 is formed throughthe body 12 parallel to the hole 30 on the side opposite the upperportion 22. The hole 36 seats a spring pin 38 which is disposed betweenthe lifting dogs 14 a and 14 b and acts to limit the degree by which theends 18 can pivot toward each other. When the lifting dogs 14 are in thelatched position, they both bear on opposite sides of the spring pin 38.

The upper portion 22 is formed with radially extending through holes 40,44 and 46, each of these holes extend transversely to the holes 30 and36. The hole 40 houses a spring 48 and, on opposite side thereofrespective locking balls 50 which together form part of a détente.

The hole 40 is of a diameter larger than the diameter of the lockingballs 50. The holes 44 and 46 accommodate spring pins 52 and 54. Inaddition to their function as explained below, the pins 52, 54 alsofacilitate attachment of the overshot to a wireline cable, as is knownin the art.

Second ends 56 of the lifting dogs 14 are formed with respective rebates58 which face each other. As seen most clearly in FIG. 2, in theassembled overshot 10, the ends 56 extend axially beyond theintermediate portion 28 and stepped face 31 of the body 12, and byvirtue of the rebates 58, are spaced radially beyond the surface of theupper portion 22.

Inwardly directed locating pins 60 are provided at the ends 56 inboardof the rebate 58 for seating opposite ends of a compression string 62.The compression string 62 is configured to bias the latching dogs 14 sothat the ends 18 are biased toward the capture position.

The locking sleeve 16 is in the form of a tube of an inner diameterslightly greater than the outer diameter of the upper portion 22 so thatthe locking sleeve 16 can rotate about the upper portion 22. A lowerportion of the locking sleeve 16 is provided with a tapered surface 64of reducing diameter leading to constant diameter circumferential wall66. The wall 66 is provided with diametrically opposed U-shaped gaps orcut-outs 68. Two first recesses in the form of radially extending holes70 are formed in the locking sleeve 16 at locations that can registerwith the position of the locking balls 50 when the sleeve 16 is in thelocked state. Two second recesses 72 (only one shown) are also formed inthe locking sleeve 16 at positions that can register with the lockingballs 50 when the sleeve 16 is in the unlocked state. The holes 70 and72 are of a diameter less than the diameter of the locking balls 50, inthis way the sleeve 16 acts to retain the locking balls 50 in the upperportion 22. Thus the combination of the spring 48, balls 50 and holes70, 72 act as a détente.

The locking sleeve 16 is retained on the body 12 by a retaining member74 which sits on the upper portion 22 behind the locking sleeve 16. Theretaining member 74 is in the form of a sleeve and provided with twopairs of holes 76 and 78 which are positioned to register with the holes44 and 46 and through which the spring pins 52 and 54 pass to attach theretaining member 74 to the body 12.

The unlocked state of the locking sleeve 16 is characterised by the gapor U-shaped cut-out 68 registering with the upper end 26 of the slot 24,and the ends 56 of the lifting dogs. In this configuration, the ends 56of the lifting dogs 14 are able to move through the gaps 68.Additionally, the holes 72 are in alignment with the holes 40 in whichcase the locking balls 50 partially extend into the holes 72 releasablyholding the locking sleeve 16 in this position. It will be appreciatedthat when the locking sleeve 16 is in the unlocked state, the liftingdogs 14 can be pivoted toward or away from each other to selectivelycapture or release a spearhead point.

In order to switch the locking sleeve 16 to the locked state, the sleeve16 is gripped either by hand or by a tool and rotated through 90°. Asthis occurs, the locking balls 50 are pushed inwardly against the biasof the spring 48 rolling out of holes 72, and the wall 60 moves behindor inside the ends 56 being accommodated within the rebates 58. Whenturned fully for 90°, the holes 70 register with the holes 40 allowingthe locking balls 50 to be pushed by the spring 48 partially into theholes 70 thereby holding the locking sleeve 16 in the locked state. Whenin this state, the ends 18 of the lifting dogs 14 can not be pivotedaway from each other from the latched position to the release position.This direction of pivoting of the lifting dogs 14, if allowed, would becharacterised by the ends 56 being pivoted toward each other against thebias of spring 62. However because the wall 60 is disposed within therebates 58, the lifting dogs 14 can not be pivoted in this direction.Thus, a spearhead point captured by the lifting dogs 14 is positivelylocked and can not be released (except for a materials failure) untilthe locking sleeve 16 is again switched to the unlocked state.

FIG. 4 depicts a second embodiment of the lockable overshot 10′. In thisembodiment, like reference numbers are used to denote like features withthe features which differ from the first embodiment being denoted withthe addition of a prime symbol. The lockable overshot 10′ includes anelongated body 12 of identical configuration and operation to thatdescribed in relation to the overshot 10 depicted in FIGS. 1–3. Thesalient differences between the lockable overshot 10′ and the lockableovershot 10 are as follows. In the overshot 10′ the ends 56′ of thelifting dogs 14 a, and 14 b are not provided with the recess 58 depictedin FIG. 1. Further, the locking sleeve 16′ has been reconfigured. Inparticular, the holes 70, and 72 of the first embodiment have beenreplaced with axially extending elongated slots 70′ and 72′. As with theprevious embodiments, these slots are disposed so as to partiallyreceive the locking balls 50 when the locking sleeve 16′ is in thelocked state and unlocked state respectively. The slots 70′ and 72′ areof a circumferential width smaller than the diameter of the lockingballs to ensure that they cannot escape therefrom. However, theprovision of the slots 70′ and 72′ allows the sleeve 16′ to also slideaxially along the upper portion 22 of the body 12. Further, an end ofthe sleeve 16′ adjacent the lifting dogs 14 has been reconfigured sothat the tapered surface 64′ has been replaced with a stepped surfacewhich leads to a shoulder 69 of constant diameter which in turn stepsdown at edge 71 to circumferential wall 66′. The wall 66′ is tapered soas to reduce in outer diameter in a direction toward the lifting dogs14.

The retaining sleeve 74 of the first embodiment is now replaced with abush 74′ having a tubular body with an increased diameter head, and isattached to the end of the upper portion 22 distant the lower portion 20by any conventional means (not shown). A spring 75 is located over thetubular body of the bush 74′ seated between the locking sleeve 16′ andthe increased diameter head of the retaining bush 74′. The tubular bodyof the retaining bush provides an upper limit to the sliding motion ofthe locking sleeve 16′

By virtue of the taper of the wall section 66′, the provision of slots70′ and 72′ and spring 75, the sleeve 16′ is able to slide axially alongthe body 12 as well as rotate about the axis of the body 12. When thelocking sleeve 16′ is in the position as depicted in FIG. 4, the wall16′ is disposed inside the ends 56′ of the lifting dogs 14. The lockingballs 50 (not shown in FIG. 4) extend partially into the slots 70′ andact as a détente to assist rotational motion of the locking sleeve 16′.The spring 75 biases the locking sleeve 16′ axially toward the liftingdogs 14 so that the wall 66′ abuts against the inside of the ends 56′.This effectively prevents the first ends of the lifting dogs 14 frompivoting away from each other to release a previously captured spearheadpoint.

By rotating the locking sleeve 16′ through ninety degrees so that thelocking balls 50 register with the slots 72′, the sleeve 16′ is switchedto the unlocked state where the ends 56′ are located within respectivegaps 68. This corresponds with the unlocked state where the first ends18 of the lifting dogs 14 can be pivoted away from each other.

By providing a taper on the wall 66′, which terminates at edge 71 of theshoulder 69, the locking sleeve 16′ also provides a visual indication ofthe wear of the lifting dogs 14. As the lifting dogs 14 wear, the wallsection 66′ is able to be pushed further in the axial direction by thesprings 75 thereby reducing the distance between the ends 56′ and theshoulder 69, when the locking sleeve 16′ is in the locked state. Thediameter of the wall section 66′ adjacent the edge 71 can be produced toa specific diameter which, when adjacent the ends 56′ provides anindication that the lifting dogs 14 are worn out and should be replaced.

Now that an embodiment of the present invention has been described indetail it will be apparent to those skilled in the relevant arts thatnumerous modifications and variations may be made without departing fromthe basic inventive concepts. For example, the retaining member 74 maybe attached to the body 12 by means other than the spring pins 52 and54, for example by use of a bolt or by way of screw threaded engagementbetween the sleeve 74 and upper portion 22. Also, other forms ofmechanism other than the détente system formed by the spring 48 lockingballs 50 and holes 70, 72 can be used to releasably hold the lockingsleeve 16 in the locked or unlocked states. Additionally, the lowerportion 20 of the body 12 can be made of a transverse section other thanoctagonal, for example, circular, square or hexagonal. All suchmodifications and variations are deemed to be within the scope of thepresent invention, the nature of which is to be determined from theabove description.

1. A lockable overshot including at least: an elongated body; at leasttwo lifting dogs pivotally mounted to said body, said lifting dogshaving respective first ends configured for latching a spearhead pointtherebetween, said first ends being pivotally moveable toward each otherto a latched position wherein a spearhead point is latched therebetweenand are pivotally moveable away from each other to a release positionwhere a latched spearhead point is released; and, a locking sleeveretained movably on said body and switchable, by movement over the bodybetween: a locked state where said locking sleeve locks said liftingdogs in said latched position and, an unlocked state where said lockingsleeve allows said lifting dogs to move to said release position, saidlocking sleeve being rotatably retained on said elongated body andswitched between said locked state and said unlocked state by rotatingsaid locking sleeve about a longitudinal axis of said elongated body. 2.The lockable overshot according to claim 1 wherein said lifting dogshave respective second ends opposite said first ends and said liftingdogs are pivotally mounted at a location intermediate said first andsecond ends to said elongated body, and at least a portion of saidlocking sleeve is disposed inside said second ends when in said lockedstate.
 3. The lockable overshot according to claim 1 wherein saidlocking sleeve is provided at one axial end with a circumferential wallformed with a gap for each of said lifting dogs wherein when in saidlocked state said wall is located inside said respective second ends andwhen in said unlocked state said second ends are located in said gaps.4. The lockable overshot according to claim 3 wherein said lockableovershot further includes détente means for releasably holding saidlocking sleeve selectively in said locked and unlocked states.
 5. Thelockable overshot according to claim 4 wherein said détente meansincludes a biased locking ball extending from an outer surface of saidelongated body and first and second recesses are formed in said lockingsleeve for partially receiving said locking ball, said first recessbeing disposed to register with said locking ball when said lockingsleeve is in said locked state and said second recess is disposed toregister with said locking ball when said locking sleeve is in saidunlocked state.
 6. The lockable overshot according to claim 5 whereinsaid circumferential wall is of a constant outer diameter.
 7. Thelockable overshot according to claim 5 wherein said circumferential wallis formed with a reducing outer diameter in a direction toward saidlifting dogs.
 8. The lockable overshot according to claim 7 wherein saidlockable overshot further includes bias means for biasing said lockingsleeve axially in a direction toward said lifting dogs.
 9. The lockableovershot according to claim 8 wherein said first and second recesses arein the form of axially extending slots.
 10. The lockable overshotaccording to claim 9 wherein said locking sleeve is further providedwith a shoulder of constant outer diameter adjacent said circumferentialwall on a side distant said lifting dogs.
 11. A lockable overshotincluding at least: an elongated body; at least two lifting dogspivotally mounted to said body, said lifting dogs having respectivefirst ends configured for latching a spearhead point therebetween, saidfirst ends being pivotally moveable toward each other to a latchedposition wherein a spearhead point is latched therebetween and pivotallymoveable away from each other to a release position where a latchedspearhead point is released; a locking sleeve retained movably on saidbody and switchable relative to said body between: a locked state wheresaid locking sleeve locks said lifting dogs in said latched positionand, an unlocked state where said locking sleeve allows said liftingdogs to move to said release position; and détente means for releasablyholding said locking sleeve selectively in said locked and unlockedpositions.
 12. The lockable overshot according to claim 11 wherein saiddétente means includes a biased locking ball extending from an outersurface of said elongated body and first and second recesses formed insaid locking sleeve for partially receiving said locking ball, saidfirst recess being disposed to register with said locking ball when saidlocking sleeve is in said locked state and said second recess isdisposed to register with said locking ball when said locking sleeve isin said unlocked state.